Dropbox

DNS setting

/etc/resolveconf/resolv.conf.d/base

nameserver 8.8.8.8
nameserver 4.4.4.4

Then

$ sudo resolvconf -u

Alternatively, you can add the following line at /etc/network/interfaces (but don't use these two methods together). Also this method requires rebooting the system (or at least, ifdown eth0; ifup eth0).